Aluminum coated steel sheets, prepared aluminum-silicon alloy coating onto cold rolled steel sheets, combine the corrosion resistance, heat resistance, attractiveness and thermal reflection properties of aluminum; and the mechanical and other physical properties of a cold rolled steel sheet itself and are widely used in automobiles, electrical appliances, building materials, and so on.
In terms of methods for meeting formability, heat resistance, discoloration resistance and oxidation resistance, there is a technique in which alloying elements are added to a steel to facilitate alloying between an steel substrate and an aluminum coating layer, thereby producing a thermally stable layer, and at the same time, retaining high heat resistance through the stable oxide layer produced on the coating layer. There is also provided a technique in which special elements are added to the steel substrate to provide the coating layer with sacrificial anti-corrosion, thereby improving the corrosion resistance of the steel substrate as well as significantly lengthening the corrosion resistance life of the entire coating layer.
A temperature allowing for the use of an aluminum coated steel sheet without the discoloration thereof is about 400° C., approximately 100 to 150° C. higher than that of a hot dip galvanized steel sheet, and, due to advances in industry, there is a need to develop components which require improved durability and heat resistance in higher temperature conditions than conventional heat resistant temperatures.
There are also a need for yield strength at high temperatures of about 600-700° C. to be above a certain level, and a need for the steel sheet to secure the adhesion of the aluminum-steel alloy layer and have good oxidation resistance, despite repetitive heat treatments at 800° C.
To meet such needs, research into the manufacturing of a heat-resistant steel sheet has been proceeding to improve high temperature heat resistant properties such as high temperature strength, oxidation resistance, discoloration resistance, and corrosion resistance. In order to achieve the objective, Japanese Patent Application Laid-open No. HEI 2-61544 discloses a method for adjusting soluble N and Al contents contained in steel, and performing a heat treatment after coating. However, there are limitations, in the addition of a process and possibility of aging due to the difficulty in adjusting the addition amounts.
In addition, Japanese Patent Application Laid-open No. HEI 8-319548 discloses a method for hot dip aluminizing in which an intermetallic compound layer having the composition of Fe, Mn, Si, Cr, and the like in specific ratios is coated onto the surface of the steel sheet in a coating bath to which Mn and Cr are simultaneously added at to conventional Al and Si coating bath components. However, there is a limitation in the method disclosed above, in that the coating bath composition should be changed, and there may be inconvenience in adjusting the composition of the coating layer surface.
In addition, Japanese Patent Application Laid-open No. 2000-290764 discloses a method for producing a hot dip aluminum coated steel sheet having excellent blackening resistance after heat treatment, in which coating is performed while adjusting the soluble N content in a steel substrate; a special chromate post-treatment is performed; a reheating treatment at about 300 to 500° C. is performed; and then Al of the coating layer and soluble N in steel are allowed to react to produce AlN at the interface between the coating layer and the steel substrate. However, this method has a limitation in that it may be complicated due to the addition of a process and there is the possibility of aggravated blackening when the strength of the steel substrate is increased or the post-treatment solution is ill-managed.
Reducing the Si content and adding Mn, Cu, and Mg elements to an Al coating bath is disclosed in Japanese Patent Application Laid-open No. 2004-238657, while Japanese Patent Application No. 2003-034854A discloses that the addition of Cr induces a change in the microstructure of the coating layer, thereby improving corrosion resistance. However, there is a limitation that the added elements in the coating bath may increase the viscosity of the surface of the coating bath such that it may be significantly difficult to adjust the concentration of Cr at a constant level and a general coating operation may not be able to be carried out at high speed.
An aspect of the present invention provides an aluminum coated steel sheet in which heat resistance and oxidation resistance of the aluminum coating layer are improved by controlling the composition thereof and forming an alloy layer at the interface between a steel substrate and the coating layer.